This is NOT an ATF-defined "destructive device" requiring the $200 NFA stamp. It is a perfectly legal firearm because it has a bore in excess of one-half inch and fires shotshells.
Dean Weingarten at Gun Watch explains in some detail with feedback from Mossberg:
I was surprised to learn that firearms that fired shotgun shells with a bore of more than .5 inches, and a barrel length less than 18 inches, could legally be purchased and possessed without all the complex regulatory burden and $200 tax required by the National Firearms Act, the Gun Control Act of 1968, and all the following changes.
The firearm pictured above is based on the Model 500 series. At the Shockwave Technologies site, there is considerable discussion about ordering a 14" front end to use to build a legal (non NFA) firearm with a barrel less than 18 inches.
©2015 by Dean Weingarten: Permission to share is granted when this notice is included. Link to Gun Watch
I would love to have one of those without jumping through the NFA hoop. These are useful weapons for civilians and have more practical applications for someone like me than other perfectly legal weapons like the Rossi mare's leg lever-action or AR-type pistols. It's not something to use for shooting skeet, but with a little practice, I am sure it could be controlled and fired with reasonable accuracy at 10 or 15 yards. Short-barreled shotguns always made sense to me - - as does basically any shotgun.
Aside from another one of my gun fetishes, the larger point that Weingarten and others make is that the ATF has a myriad of regulations that are often contradictory in some aspects. Why is this an acceptable weapon if there was never a shoulder stock installed on it at the factory? If there was a factory shoulder stock and someone modified it afterward, it is a destructive device. It's a morass.